The main problem that I can forsee is that water from coal mines tends to be very dirt and polluting. It is claimed that it will be a closed system with the same dirty water being reused, but I expect that some will escape and upset someone._________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"

There was some thought given to developing an underground pumped storage scheme in London's Docklands. This was about 15 years ago. It came to nothing for various reasons. One of the attractions of the scheme would have been that it was right in the middle of a major load centre.

AFAIK the docklands scheme was to have been compressed air, not water.
Water for pumped storage requires two reservoirs at significantly different heights, such as the existing scheme in Dinorwic, Wales, or the proposed German one.

The docklands area is flat and it seems unlikely that two reservoirs at substantialy different heights could be found.

Compresed air storage requires only a single pressure tight reservoir, man made pressure tanks are not normally viable owing to the volune required.
Empty salt mines, oil or gas reservoirs are sometimes used..

Either technology involves substantial capital expense and energy losses.
It is normally more economic to use other ways of matching supply and demand._________________"Installers and owners of emergency diesels must assume that they will have to run for a week or more"

It is normally more economic to use other ways of matching supply and demand.

One wonder for how much longer though?

The "first line" of defence is a small voltage drop, but what cares about that now? (Almost) All modern appliances will simply draw more amps as voltage drops, all that springs to mind is resistance heating, but how much of load is that now?

The next stage would be interupting big users on interuptable contracts, but thats again getting much more difficult. Big Arc Heaters in steel mills are the usual culrpits, but how many steel mills are left?_________________I'm a realist, not a hippie

AFAIK the docklands scheme was to have been compressed air, not water.

I had a very slight involvement with the assessment process, and it was water. The top reservoir was obviously the Thames/sea, the lower was an excavated cavern. The head was 300 m if I recall correctly.

There are a couple of compressed air storage power stations in operation; one is Huntorf in Germany. From what very little I know of these, they're not as effcient as a hydro pumped storage system and they're not as responsive - they take a long time to start up and stop.

You say "It is normally more economic to use other ways of matching supply and demand." Well, that's absolutely correct and as far as pumped storage in the UK is used and operated it is never viewed as the main money-earner for PS. Load shifting by PS does occur, of course, but only as a bi-product of PS providing other more lucrative and important services such as response and reserve.

It's for this reason that you'll find PS used most heavily in grid systems which are either islanded or peninsular. Those grid types have far greater issues with grid system stability.

However, DominicJ does have a point. In this stupid new world of increasing percentage renewables, load shifting is now becoming an issue, and we might well build more PS just to load shift. SSE is building two PS on the Great Glen. Goodness knows how they justify these economically, but then no one understood how they were going to make money on Glendoe (and they aren't now, since the main tunnel collapsed).

PS can provide many services to a grid: Response, reserve, black station start, peak-lopping (aka load shifting), reactive control are common grid services. They can also contract privately for contracts for differences as a hedge against spikes in spot price excursions.

But of all these, load-shifting is the weakest profit line mainly because you need a price differential between pumping and generating large enough to compensate for the PS system efficiency (75 %). The trick is to try and sell two services on the same chunk of stored water - such as load-shifting blended with a bit of response (as in part-load response).